Polystyrene is produced in an expandable granular form referred to as polystyrene "beads." The beads have a closed cellular structure and contain an expanding agent such as pentane. The beads may be formed into integrated bodies, such as sheets or slabs by a molding process in which steam and heat are applied to cause the beads to soften, expand and integrate. In most applications, the beads are subjected to a pre-expansion before being formed into the molded bodies, the beads being only partially expanded during the pre-treatment. Polystyrene insulating board as presently produced from such beads usually does not contain any additional binders or fillers. However, the patent literature contains some descriptions of such composites.
In the process described in U.S. Pat. No. 3,154,604, pre-expanded polystyrene beads are combined with a thermoplastic polymeric bonding agent that is adapted to flow at a temperature lower than the expansion temperature range of the beads. This mixture is mechanically preformed at a temperature at which the thermoplastic is flowable but below a temperature at which the beads expand. Thereafter, the preformed mass is subjected to heat to expand the beads.
The U.S. Pat. No. 3,251,916 discloses the formation of composite bodies from expanded polystyrene and a polyurethane binder. The polystyrene beads are expanded before being mixed with the binder.
U.S. Pat. No. 4,079,170 describes the impregnation of a pre-formed sheet of expanded polystyrene with a wax composition, the liquified wax being forced into the sheet under pressure and hardened by cooling.
U.S. Pat. No. 3,577,363 discloses a process for combining urea-formaldehyde resin with polystyrene beads. The mix is formed in molds for heating in stages of increasing temperature. The process results in opening the cells of the polystyrene beads.
U.S. Pat. No. 4,256,803 describes the combining of phenolic resin with polystyrene. The resin contains a foaming agent. After forming the mix, a stage heating procedure is employed. Polystyrene is one of a number of fillers that can be employed.
U.S. Pat. No. 3,630,787 describes a process in which polystyrene granules are precoated with an epoxy resin and thereafter formed into molded bodies. In the molding process, the mix is compressed sufficiently to press the binder into and around the beads. The final curing stage can be carried out in a furnace or by exposure to a high-frequency alternating electric field (viz. microwave heating).
One of the known disadvantages of expanded polystyrene insulation is that it is a flammable material which can contribute to fire spreading. The incorporation of fire retardant chemical has been proposed, but such chemicals increase the cost of the insulating material. Further, the incorporation of fire retardant chemicals may interfere with the self-bonding of the polystyrene beads during the manufacture of the board, thereby reducing the mechanical strength of the insulation and increaings its friability. Heretofore, no method has been proposed whereby the fire resistance of polystyrene bead insulation can be appreciably increased at minimal additional cost, and with substantial improvement in other desirable properties.
The manufacture of polystyrene insulating board by integration and expansion of polystyrene beads without the use of a binder results in molded board products requiring shaping by trimming. Large quantities of such trimmings, referred to as "polystyrene scrap", accumulate as a waste by-product in connection with polystyrene board manufacturing operations. Presently, there is little use for such polystyrene scrap. It is employed to some extent as a packaging material, and other uses are being sought. Most of the polystyrene scrap is understood to be disposed of by burning.
Foam plastic scrap is also available from other sources. Foam plastic bodies which are molded and then cut or trimmed are made from a number of other plastics besides polystyrene. These include polystyrene-polyethylene copolymers, polyethylene, acrylonitrile butadien styrene (ABS) copolymers, polyurethane, polypropylene epoxy, polyvinylchloride, polyimides, and silicones. All of these foamed waste materials have relatively little use at present and tend to be materials of low fire retardancy.